The following discussion is intended for explanatory purposes only and is not to be considered an admission any of the information is legal “prior art.” In the work and development of the magnetostrictive actuator for use in a programmable diesel fuel injector in accordance with U.S. Pat. Nos. 7,255,290, 8,113,179, and 8,418,676, all of which are incorporated in their entirety herein, additional opportunities to improve upon the actuators have also been identified.
Looking first at the example where an actuator is used in a diesel fuel injector, it is understood that pollutant formation is controlled by combustion complexities. Before the fuel burns, certain physical and chemical processes must occur to prepare the mixture of air and fuel vapor. Much technical literature and many patents disclose that metering very quick jets or pulses of standard number two liquid petroleum diesel fuel helps to reduce pollutants. Injecting a fuel volume of order one cubic millimeter prior to a main charge injection is advantageous in suppressing pollutant formation. As used here, “prior to a main charge injection” means that the time delay between the small and main charges should be as short as possible.
A preferred actuator within the fuel injector exhibits the highest possible speed, control of that speed, and minimum delay as needed between pulses. For certain conditions, more than one small charge may be further advantageous. This performance combination has been difficult to achieve in practice because actuator technology lagged. Much creative and ingenious innovation has gone into improving control over diesel fuel injection. Ultimately, these efforts are limited by the physics of the two main electrical control technologies used to date: solenoids and piezo-electric ceramics, hereinafter piezo. Solenoid injectors date at least as far back as Gaff in 1913 while piezo injectors date at least from Bart in 1977. Thus, both solenoids and piezo have had the benefit of sustained attention to their limitations. Well into the piezo injector era, Benson et al in 2008 show that piezo has yet to fully replace solenoid technology.
Solenoids offer durability, but are unsuitable for continuous control. Their key characteristic is that the mechanical motion can never be proportional to electrical input. While durable and reliable, precise actuator control remains elusive and thus neither intelligible speech nor ideal fuel rate shapes nor quick jets with minimal delay can be reproduced by the solenoid. By its operating principle, when a magnetic flux above a threshold value crosses an air gap, its two poles accelerate toward each other, closing the gap until, eventually, they impact each other and, depending on design details, bounce back. The force that accelerates the two poles is inversely proportional to the square of the gap between them, making velocity or position control difficult. Thus, the solenoid is either open, closed, bouncing, or transitioning between these states at a more or less uncontrollable rate.
Piezos offer speed and infinitely adjustable displacement within their range, permitting continuous control. The key feature of this technology is that mechanical expansion is proportional to applied voltage, within limits. Piezo force and displacement are akin to thermal expansion except electrically controllable and much, much faster. Piezos can be used to reproduce intelligible speech or to rate shape injected fuel, but only for a while. Their inherent critical defect is susceptibility to performance degradation as noted in U.S. Pat. Nos. 5,875,764, 7,159,799, and 7,262,543, MIL-STD-1376, and Cain et al, among many references. This degradation or aging is the Achilles heel of piezo technology, disabling its use in a durable, continuously controllable, fast diesel injector. When lightly loaded to get reasonable life, piezos can offer a telegraph-style ON-OFF speed improvement over solenoids, enabling the faster and smaller multiple pulse injections in use to reduce in-cylinder formation of diesel emissions. Despite its speed and proportionality, limiting piezo to telegraph-like behavior to get a reasonable working life makes this approach less than ideal for rate shaping fuel injection.
The piezoelectric ceramic must be “poled” to operate. In context here, expansion requires an electrical input of only one polarity. If a reverse voltage of the same magnitude were applied to the piezoelectric ceramic, it is likely to be rendered inoperable by depoling. The forward voltage cannot exceed a threshold. There is thus a need for an actuator that is both durable and offers continuous control.
The US Navy developed an intermetallic alloy of iron and the rare earths terbium and dysprosium for sonar—it is the magnetostrictive equivalent of piezoelectric ceramics. The alloy couples a magnetic input to a mechanical output. It offers speed, infinitely adjustable displacement within its range, and the durability to survive on an engine cylinder head. The key feature of this technology is that mechanical expansion is proportional to the current sheet circulating around it, regardless of circulation direction. Magnetostrictive displacement and force are akin to thermal expansion except magnetically controllable and much, much faster as noted in Dapino et al and Faidley et al. A magnetostrictive actuator employing this alloy can reproduce intelligible speech or adaptably and quickly rate shape injected fuel without a durability limit.
The quantum mechanical origin of the magnetostrictive effect in the rare earth/transition metal alloy guarantees the survival of the effect itself. The effect does not fatigue. Alloy constituent proportions control the magnitude of the effect with respect to temperature, where the effect diminishes as temperature rises but returns fully as temperature falls. High field does not degrade the alloy. Thus, there is a need to address the use of such mangetostrictive materials at cold temperatures, such as those typically experienced by diesel trucks traveling throughout the continental United States.
The energy density of the magnetostrictive alloy is its source of high speed and force capabilities. These key characteristics are packaged into an alloy which also has additional limitations. In particular, the actuator can fracture its magnetostrictive rod under certain circumstances. For example, the high speed and high force of the actuator can accelerate its mechanical load. Depending on the details of a design, its fabrication, and its operation, after the initial acceleration the rod may decelerate faster than its load causing a gap to appear. The load will eventually close that gap and re-establish contact with the rod. However, that may be by impact, with the attendant shock reloading of the rod.
Impact shock can cause rod fracture. The likelihood of fracture is affected by impact magnitude, foreign matter between impacting surfaces, misalignment between impacting surfaces, and gap width between the impacting surfaces. The magnetostrictive effect itself, being of quantum mechanical origin, is unaffected by impact or fracture. In other words, the rod will continue to function to some degree. Another circumstance that can cause impact shock is the presence of an obstacle within the range of the actuator's displacement. Depending on the speed at which it occurs, contact with the obstacle may cause impact shock. There is thus a need to address and attempt to minimize the tendency of both impact and potential fracture to enable employment of the full speed and power of this actuator.
Further, if periodic excitation, such as multipulse fuel injections, coincides with the natural frequency of the actuator, a standing wave will develop within the magnetostrictive rod. The standing wave is comprised of two separate elastic waves, traveling in opposite directions and superposed onto each other. The speed at which the elastic wave travels is determined by the alloy density and by its elastic modulus. The waves reflect at the rod end discontinuities. The standing wave causes a geometric location within the rod to experience the highest tensile stress within the entire rod, which could also fracture the rod. There is thus a need to widen the actuator operating range without exceeding its rod tensile limit.
Additionally, precision fuel injection requires fine control, which in turn requires calibration. For continued good fuel economy and low emissions, the continuously variable actuator and injector must always operate precisely. Overall precision of a diesel injector is affected by many influences. Machining tolerances must fit in a range yet be economical to fabricate. The continuously variable actuator must move from one precisely known location to another precisely known location in a precise amount of time. Impact of solid components causes fatigue cracking, fretting wear, and undesirable dynamics. Depending on the design, mechanism wear may thus occur, particularly with frequent and consistent impacts between the rod and other components.
Thus, it is important to know when an impact occurs. That is, it is desirable to know how long an electrical pulse can be applied to the magnetostrictive actuator. This time will drift within an individual actuator due to component wear and change in external influences. This time will also vary between individual actuators due to fabrication differences. Thus, it is desired that a displacement calibration reference be obtained from the actuator during its life cycle to further minimize the potential for fracture of the rod and damage or wear to other components of the system.